The basic concept is explained in Drawing on an image with TikZ. Add the image in a named TikZ node, and place stuff relative to that. (When you know the text should be in a corner there is no need to use the fancy stuff with the scope environment seen in the answers to that question.)
To allow for line breaking, set the text width and/or the align of the node used to hold the text, as in the example below.
In that case the node will act sort of like a minipage I think, so a footnote will end up at the bottom of the node, not the page. Using \footnotemark and \footnotetext is a workaround for that, though with floating figures (if you're using that) that leads to problems of their own.

\documentclass{article}
\usepackage{tikz,graphicx}
\begin{document}
\begin{tikzpicture}
\node (img) {\includegraphics[width=8cm]{demo}};
\node [below left,text width=3cm,align=center] at (img.north east){Lorem ipsum dolor sit amet consectetur and then a bunch more stuff that no one remembers.\footnotemark};
\end{tikzpicture}
\footnotetext{Sure about this?}
\end{document}
tikzpicture, and that might in turn change where on the page the image node ends up. Depending on how thetikzpictureis aligned on the page, and where the text is in relation to the image. For example, if thetikzpictureis placed in a normal paragraph, and the text is placed to the left of the image, then the image by necessity will go further right on the page. – Torbjørn T. Jan 12 '21 at 18:38